Elastically or plastically highly extensible (strippable) self-adhesive tapes which can be redetached without residue or destruction by means of extensive stretching substantially within the plane of the bond are known from U.S. Pat. No. 4,024,312, DE 33 31 016, WO 92/11332, WO 92/11333, DE 42 22 849, WO 95/06691, DE 195 31 696, DE 196 26 870, DE 196 49 727, DE 196 49 728, DE 196 49 729, DE 197 08 366, DE 197 20 145, WO 99/31193, and WO 99/37729.
They are frequently employed in the form of single-sidedly or double-sidedly pressure-sensitive adhesive sheet strips (adhesive tape strips, adhesive strips), which preferably have a noriadhesive grip tab region from which the process of the detachment is initiated. Particular applications of such self-adhesive tapes can be found, inter alia, in DE 42 33 872, DE 195 11 288, U.S Pat. No. 5,507,464, U.S Pat. No. 5,672,402, and WO 94/21157. Specific embodiments are described, for example, in DE 44 25 587, DE 44 31 914, WO 97/07172, DE 196 27 400, Wo 98/03601, and DE 196 49 635, DE 197 20 526, DE 197 23 177, DE 297 23 198, DE 197 26 375, DE 197 56 084, and DE 197 56 816.
Preferred areas of use of the abovementioned adhesive sheet strips include, in particular, the residuelessly and nondestructively redetachable fixing of light to moderately heavy articles in the home, workplace, and office segments. In these applications they replace conventional fastening means, such as hooks, pins, thumbtacks, nails, screws, conventional self-adhesive tapes, and liquid adhesives, to name but a few. Key to the successful use of the abovementioned adhesive sheet strips is not only the possibility of residueless and nondestructive redetachment of bonded articles but also their quick and easy bonding, and the secure hold they provide for the envisaged period of bonding.
In accordance with the abovementioned patent documents, pressure-sensitive adhesive compositions suitable for such products include in particular those based on natural rubber/resin mixtures, synthetic rubber/resin mixtures, and acrylate copolymers. In practical application, however, it is pressure-sensitive adhesive compositions based on styrene block copolymer/resin mixtures that have taken over the market. For instance, the products tesa® Power-Strips®, tesa® Power-Strips® mini, and tesa® Poster-Strips from Beiersdorf AG, Hamburg (D), Command® Adhesive from Minnesota Mining and Manufacturing Co. Inc., St. Paul (US), and Plastofix® Formule Force 1000 from Plasto (F), all utilize a pressure-sensitive adhesive composition based on styrene block copolymer/resin mixtures.
Also sold commercially are devices made of plastic, normally bonded in the form of hooks with abovementioned double-sidedly pressure-sensitively adhesive products and redetachable without residue or destruction by extensive stretching in the bond plane.
Devices of this kind are described, inter alia, in DE 42 33 872 and WO 94/21157. DE 42 33 872, for instance, describes a redetachable self-adhesive hook equipped with a pull-release adhesive sheet (available commercially, inter alia, under the designation “tesa® Powerstrips® system hook”). Devices which can be used in this way are composed of only one plastic part or have a modular composition comprising two or more functional parts. The abovementioned system hook is composed of 2 functional parts, a baseplate and a mountable hook molding.
In the course of practical use of such devices, however, problems may occur, especially during redetachment by extensive stretching of the adhesive sheets used. At the extensional stress which occurs in this procedure, the effect of debonding is of substantial significance, in addition to a sufficiently high tensile strength. It is also necessary to ensure that the pulling of the adhesive strips takes place preferentially in the bond plane.
One possibility of circumventing the restriction just described can be found in EP 832 588, in which a redetachable self-adhesive device is provided with regions of low static friction and sliding friction. For a device to have such a construction, however, complex production technologies are needed (2-component injection molding and many more).
An additional disadvantage with the solutions known to date is that the pressure-sensitive adhesive compositions used which are able to meet the typical profile of requirements for the applications described, are restricted to a relatively large extent in the degree of freedom of the formulations and in the selection of the base materials. In many cases, high-grade modern adhesive formulations are unsuitable for the described redetachment mechanism by the bonding on stretching in the bond plane, since debonding is not sufficient. In this case the static friction and sliding friction, especially in edge regions of the bonded devices, are too high, and the adhesive strip then tends to tear, so that traceless redetachment of the device is no longer possible.
In addition, misapplications may occur for the inexperienced user if she or he fails to perform the stretching operation in one step and instead carries out stretching and release a number of times, in stages, so that in moments of release the adhesive strip may become adhesively bonded again, particularly to the edge regions of the device. This renewed bonding under simultaneous extensional stress then leads in many cases to the adhesive strips becoming torn off, as a result of additional frictional forces which occur.
Effected to a particularly great extent by these application areas are adhesive compositions which, in principle, have a tear propagation force which is much less than the tear force. Under the above-described influences of rebonding and reduced debonding such adhesive compositions are subject to very small instances of damage superficially, which owing to the low tear propagation force may lead very quickly to tearing of the adhesive strip.
Applications of these adhesive films, redetachable by extensive stretching, for decoration purposes are accumulating to an increasing extent. In such applications, visual aspects are much more significant. In these cases, particular value is placed by the users on the fact that the adhesive strips and devices (e.g., hooks) used appear visually in as unobtrusive a way as possible in relation to the decoration that is to be affixed. The prior art does not describe how a device (e.g., hooks, baseplates) is to be modified in such a way that the favored redetachment characteristics and the described optical neutrality function in combination. Thus, DE 197 29 706 describes regions which reduce static and sliding friction and which are composed of polymeric materials. However, these regions are normally not neutrally colored and/or not transparent in terms of their visual appearance, so that according to this description a desired lack of visual obtrusiveness in the user's applications cannot be achieved.
It has in fact been found that the devices or plastic parts, especially hooks, of this kind that exist have not been paid the attention appropriate to them, either in the patent literature or in practical use of the various commercially available products. Indeed, despite all of the efforts made to date, tearing occurs again and again during detachment. Quite evidently, the problem has not been fully recognized. It is found that the adhesive sheets present on the market to date, which am redetachable without residue or destruction by extensive stretching in the bond plane, do not possess a device rendering them capable of very largely retaining tear-free redetachment even under relatively disadvantageous debonding characteristics and application influences. Equally, in many cases, the desired, visually neutral appearance during application is not achieved.
It was an object of the present invention to remedy this situation and, in particular, to be able to offer the user a product of consistently high and uniform quality without him or her when using said product being disappointed in terms of the performance of a bond made using it or its subsequent parting, in particular as a result of tearing.